The present invention relates to improvements to tensioners used in power transmissions which comprise a belt and at least two pulleys, that is a driving pulley and a driven pulley, respectively.
In such power transmissions, the tensioner acts with its pulley on the back of the belt to tension it to a value that assures normal working of the power transmissions.
It is known to use a tensioner for driving belts which comprise a stationary structure, a lever arm supported by the stationary structure and movable with respect to the latter, a pulley associated with the arm, a first mechanical or torsion spring means between the stationary structure and the arm to move the arm with respect to the stationary structure and to urge the pulley against the belt, frictional dampening means associated with the stationary structure and the arm to dampen the movements of the arm rotating with respect to the stationary structure in both rotation directions. The stationary structure comprises a shaft having a longitudinal axis mounted in a stationary manner with respect to the movements of the rotating arm and the arm has a portion carried by the shaft so as to rotate relative thereto.
A tensioner as set forth above is described in greater detail in U.S. Pat. No. 4,473,362, the disclosure of which is hereby incorporated by reference.
The tensioner of U.S. Pat. No. 4,473,362 is provided with a torsion spring having at the ends thereof a rotating arm and a stationary structure and a plastic spring holder ring with cylindrical walls and supported by the lower portion of the stationary structure coaxially to the shaft. The lever arm lengthens considerably starting from the tensioner shaft over its radial path which is determined by the torsion spring and carries at its free end an idle pulley.
During operation one end of the torsion spring for the rotation of the lever arm is clamped on the lateral wall of the spring holder element which is urged against rotating parts of the tensioner giving rise to a frictional dampening.
In this tensioner the dampening is proportional to the force of the torsion spring.
Tensioners having the above mentioned characteristics are known, in which however the dampening has constant values independently of the values of the forces of the mechanical spring means that urge the tensioner pulley towards the belt.
Such a tensioner is described in the U.S. Pat. No. 4,596,538 the disclosure of which is hereby incorporated by reference.
In this tensioner, a flat torsion spring wound coaxially to the shaft is arranged in a first plane and a dampening device, formed by two plates opposite to each other and second mechanical spring means, is arranged in a second plane lying below the first plane.
One of the plates is mounted coaxially to the shaft, rotates with the arm and is axially movable relative to the second plate integral with the stationary structure.
The second spring means comprises a wavy spring that urges one of the plates against the other to determine the desired frictional value during the rotation of the arm and to compensate for wear appearing in the form of dust due to the friction between the plates.
The tensioner arm extends considerably over the radial encumbrance of the flat torsion spring and the underlying damper and carries at its end an idle pulley for tensioning the belt.
A further tensioner having a constant frictional dampening is described in U.S. Pat. No. 4,971,589 the disclosure of which is hereby incorporated by reference. This tensioner provides a flat torsion spring wound coaxially to the shaft and on the shaft itself.
The stationary structure provides a cup-shaped envelope having a base to which the shaft is secured and lateral walls ending in an outwardly flaring shaped with a conical surface portion having an inclination converging towards the shaft. The arm has a conical surface complementary to that of the envelope. A layer of suitable anti-friction material is arranged between the two conical surfaces. The second mechanical means is represented by a wavy spring that, from the tensioner upper part, urges the conical surface of the arm against that of the envelope.
The conical surfaces are adopted to oppose a "cocking" phenomena, i.e. the inclination of the tubular portion of the arm on the shaft. Such "cocking" is due to the fact that the horizontal force transmitted by the belt to the pulley is applied at a certain distance from the lower portion to which the shaft is secured thus originating a bending moment with a resulting inclination and a consequent wear of the anti-friction material layer between the shaft and the portion of the arm rotating around the shaft. In this tensioner the dampening is determined by the friction due to the movements relating to the rotation between said conical surfaces at the periphery of the envelope and assumes high values due to the sensible mean radius of the conical surfaces measured with regard to the axis of the shaft.
A further tensioner is known from U.S. Pat. No. 4,826,471, the disclosure of which is hereby incorporated by reference, which comprises first mechanical spring means formed by a torsion helical spring mounted coaxially to the shaft between the arm and the stationary structure.
This tensioner makes use of a dampening device comprising two cams having inclined surfaces cooperating with each other and second mechanical spring means formed by a compression spring. One of the cams is mounted on the lower base of the arm and the second cam having an inclined surface directed towards the first inclined surface is mounted coaxially to the shaft on which it can slide only axially through a guide which is longitudinally notched to the shaft.
The second cam is urged against the first one by the compression spring disposed with its lower end on a stationary base, coaxially to the shaft and inside the torsion spring. The arm lengthens over an envelope of space defined by the two springs.
During operation, in relation to a predetermined direction of rotation of the arm, one of the cams goes up with its inclined surface towards the other one, compressing the spring which when unloading its force obliges the cam to go down towards the other one with a movement of the arm against the back of the belt.
In practice, in this tensioner the torsion spring and the compression spring act as they were in parallel to tension the belt. The working of the dampening device is unidirectional. The tensioners of the state of the art set forth above also have a dampening effect independent of the dampening device. This independent dampening is due to the friction between the shaft and the portion of the arm in rotation around the shaft since the force of the belt acting on the pulley at the end of the arm is balanced by an equal and opposite reaction directed by the portion of the arm in rotation around the shaft towards the shaft.
Consequently, the tensioners of the state of the art provide a dampening, even if minimal, due to the essential structural parts and additional dampening devices that can be grouped in two types different from each other that is, a first type in which the dampening can depend on the force of the spring or also unidirectionally and a second type in which the friction is constant for both the directions of rotation of the arm.
The present invention includes dampening devices of the second type.
The tensioners provided with devices of the second type have in several of their embodiments frictional surfaces exposed to the outside with the possible risk of embedding particles of the surrounding environment and consequent risk of jamming.
Moreover said tensioner are based on dampening devices which substantially make use of very large frictional surfaces and heavy compression loads with a consequent and not indifferent wear-in time with formation of dust and risks of jamming between the frictional surfaces in the relative rotary motion.